feat: add run task identity primitives (#19047)

## Stack

This is PR 1 of the simplified HAI single-run-task stack:

- [#19047](https://github.com/openai/codex/pull/19047) Agent Identity
assertion and task-registration primitives, including the shared
run-task helper used by existing Agent Identity JWT auth.
- [#19049](https://github.com/openai/codex/pull/19049)
Disabled-by-default ChatGPT auth opt-in that provisions/reuses persisted
Agent Identity runtime auth and its single run task.
- [#19051](https://github.com/openai/codex/pull/19051) Run-scoped
provider auth that uses one backend-owned task id for first-party
inference and compaction requests.

[#19054](https://github.com/openai/codex/pull/19054) collapsed out of
the active stack because the simplified design no longer needs a
separate background/control-plane task helper.

## Summary

The simplified POC shape is one backend-owned task per Agent Identity
run. This PR makes the first layer match that final shape directly
instead of introducing task targets, caller-owned external task refs, or
intermediate wrappers that later PRs would need to undo.

What changed:

- keeps the `AgentAssertion` wire payload as `agent_runtime_id`,
`task_id`, `timestamp`, and `signature`
- exposes `register_agent_task` as the single task-registration helper
for both existing Agent Identity JWT auth and the ChatGPT-registration
path added later in the stack
- makes task registration send only the signed registration timestamp;
the backend owns the returned opaque task id
- removes the unused target/task-kind/external-task-ref surfaces from
`codex-agent-identity`
- keeps Agent Identity JWT JWKS lookup separate from agent/task
registration URL derivation
- updates Agent Identity JWT auth to register one run task during auth
construction and share that task across cloned auth handles

This PR intentionally does not enable ChatGPT-derived Agent Identity.
That opt-in and config gate are added in the next PR.

## Testing

- `just test -p codex-agent-identity`

codex-rs/cli/src/main.rs

@@ -1731,9 +1731,12 @@ async fn load_exec_server_remote_auth_provider(
         let agent_identity_jwt = read_codex_access_token_from_env().ok_or_else(|| {
             anyhow::anyhow!("CODEX_ACCESS_TOKEN is required when --use-agent-identity-auth is set")
         })?;
-        let auth =
-            CodexAuth::from_agent_identity_jwt(&agent_identity_jwt, Some(&config.chatgpt_base_url))
-                .await?;
+        let auth = CodexAuth::from_agent_identity_jwt(
+            &agent_identity_jwt,
+            Some(&config.chatgpt_base_url),
+            /*agent_identity_authapi_base_url_override*/ None,
+        )
+        .await?;
         return Ok(codex_model_provider::auth_provider_from_auth(&auth));
     }